Reaction and distillation device and etherification process

ABSTRACT

The invention relates to a process for preparing at least one aliphatic monoalcohol, generally in excess, and at least one olefin. The process is performed in a distillation and reaction device having a reaction section and a distillation-reaction section. The distillation-reaction section has a distillation zone with at least one means for circulation of the effluent from the reaction section to the distillation zone. Effluent from the reaction section is circulated to the distillation zone. The distillation-reaction section also has at least one distillation-reaction zone having at least one catalyst bed. The distillation-reaction zone is distinct from the distillation zone and provides at least partly the reflux of the distillation zone. The distillation-reaction section can also have at least one complementary reaction zone containing at least one catalyst bed.

This is a divisional, of prior application Ser. No. 08/900,878 filedJul. 25, 1997 now U.S. Pat. No. 6,159,344.

FIELD OF THE INVENTION

The present invention relates to a reaction and distillation device andto a process for preparing a tertiary alkyl ether by reaction of atleast one olefin with an aliphatic monoalcohol, generally in excess.

BACKGROUND OF THE INVENTION

It is well-known to prepare tertiary alkyl ethers by reacting aniso-olefin generally contained in a hydrocarbon fraction with analiphatic alcohol, generally used in excess, in the presence of an acidcatalyst, for example sulfuric acid, hydrofluoric acid, aluminiumchloride or boron fluoride, or in the presence of carbonaceous mattercontaining sulfonic groups (−SO3H), for example sulfonated coal,sulfonated phenol-formaldehyde resins, sulfonated coumarone-indenepolymers or preferably sulfonated styrene-divinylbenzene copolymerresins or other compounds, notably mineral compounds, comprisingsulfonic groups (for example sulfonated polysiloxanes).

It has been known for a long time that the reaction between an aliphaticmonoalcohol and a tertiary olefin is balanced and that it is difficultto obtain iso-olefin conversion coefficients with a high purity andyield. Conventional processes such as those described for example inHydrocarbon Technology International, Autumn 95, p. 21-27, comprise oneor more reactor(s) for etherification to tertiary alkyl ethers followedby at least one fractionating zone, generally a distillation zone, whosebottom product is ether containing the lowest possible amount ofmonoalcohol(s).

This is the reason why the prior art recommended, in order to improvethe performances of this synthesis, to add a complementary reactionsection to the main reactor, as described for example in patent U.S.Pat. No. 5,364,975 in the name of the applicant. In this patent, thecomplementary reaction section is included, according to a preferredembodiment, in the reflux device of the fractionating section. It hasalso been proposed, for example in patent U.S. Pat. No. 4,503,265 or inpatent applications WO-A-93/19,031 and WO-A-93/19,032, to draw off aproduct from an intermediate tray of the fractionating section, to feedthis product into a complementary reaction section and to feed theproduct from this complementary reaction section back into thefractionating section at a level below the draw-off level. The drawbackof this embodiment is that it disrupts the smooth running of thedistillation process in the fractionating section. A process of the UOPCompany known as Ethermax, wherein the effluent from the mainetherification section is fed into a distillation-reaction zone, hasalso been described for example in Hydrocarbon Processing, March 1995,p. 114. As shown hereafter in a comparative example, this process hasthe drawback of requiring a column of very great height to obtain a veryappreciable improvement of the global performances. These conventionalprocesses for preparing tertiary alkyl ethers will be describedhereafter in connection with FIGS. 1, 2 and 3.

One of the objects of the invention is to overcome the main drawbacks ofthe processes described in the prior art and to propose several ethersynthesis embodiments allowing to maximize global conversion of theiso-olefins contained in hydrocarbon cuts.

The feed consisting of a mixture of C4, C5, C6 or C7 hydrocarbonscomprising iso-olefins and at least one aliphatic monoalcohol generallyused in excess is fed into the main reaction section represented byreactor R1 in FIGS. 1 to 9. The mixed reactants are brought into contactwith an acid catalyst.

The product from this reaction section R1 is fed into a distillationzone represented by column F1 in FIGS. 1 and 2 and by column F2 in FIG.3. It is distilled in this column in order to produce, at the bottom,through line 9, a tertiary alkyl ether containing the lowest possibleamount of monoalcohol(s), and at the top, through line 2, a mixture ofreactive and non reactive hydrocarbons and of aliphatic monoalcohol(s)carried over by azeotropy. This effluent flowing out through line 2 iscondensed in condenser E1 and collected through line 3 in drum B1 priorto flowing into pump P1 through line 4.

In the instance schematized in FIG. 1 patent U.S. Pat. No. 5,364,975),part of the effluent leaving pump P1 is fed through line 10 into areactor R2 referred to as finishing reactor, whose effluent is fedthrough line 12 into column F1 as reflux and the rest of the effluentflows off as distillate through line 6.

In the embodiment schematized in FIG. 2 (patent U.S. Pat. No.4,503,265), a product is drawn off from an intermediate tray of thedistillation column between the effluent introduction point ofetherification reactor R1 and the top of this column, and this productis fed through line 10 into a reactor R2 referred to as finishingreactor, whose effluent is fed through line 12 into column F1 at a levelbelow the level of the draw-off point. In the instance schematized inFIG. 2, part of the effluent flowing out of pump P1 is fed through line5 into column F1 as reflux and the rest of the effluent flows off asdistillate through line 6.

In the embodiment schematized in FIG. 3 (Hydrocarbon Processing, March1995, p.114), the effluent of the reaction section R1 is fed into adistillation-reaction column beneath the first catalyst bed and part ofthe effluent flowing out of pump P1 is fed through line 5 into column F2as reflux and the rest of the effluent flows off as distillate throughline 6.

However, such a process requires in the instance schematized in FIG. 3 agreat number of reaction zones in distillation-reaction column F2 inorder to obtain high conversions into iso-olefins and consequently acolumn of great height. In fact, obtaining high conversions requires adistillation-reaction column containing many reaction zones. Insofar asone of these zones alone occupies about 3 meters in height in thedistillation-reaction column, the final height of this column quicklybecomes limitative. It would therefore be interesting to have such ahigh conversion to iso-olefins while keeping a limited number ofreaction zones in the column and/or to limit the height of the column.This is one of the objectives of the invention as described hereafter inconnection with FIGS. 4, 5, 6, 7, 8 and 9.

DESCRIPTION OF THE INVENTION

The present invention thus relates to a distillation and reaction devicecomprising:

a) a reaction section,

b) a distillation-reaction section comprising a distillation zone,comprising at least one means intended for circulation of the effluentfrom the reaction section to said distillation zone, and at least onedistillation-reaction zone, comprising at least one catalyst bed, saiddistillation-reaction zone being distinct from the distillation zone andproviding at least partly, preferably for the most part most preferablypractically entirely the reflux of the distillation zone.

The device according to the invention preferably comprises at least onemeans intended for circulation of the effluent between the top of thedistillation zone and the bottom of the distillation-reaction zone andat least one means intended for circulation of the effluent between thebottom of the distillation-reaction zone and the top of the distillationzone.

The device according to the invention preferably also comprises at leastone means for feeding the reaction section and at least one means fordrawing off product at the bottom of the distillation zone.

According to one of the embodiments of the invention, independent or notof the previous embodiments, the device according to the invention issuch that the distillation-reaction zone comprises at least one refluxdevice, which generally comprises at least one draw-off means situatedat the top of the distillation-reaction zone, feeding at least onecondensation zone, at least one means intended for circulation of partof the effluent from said condensation zone towards the top of thedistillation-reaction zone and at least one means for drawing off(generally outside the distillation-reaction section) another part ofthe effluent from the condensation zone.

A preferred implementation of the device according to the invention ispreferably such that said device is characterized in that saiddistillation-reaction section comprises at least one complementaryreaction section, comprising at least one catalyst bed. Such a preferredimplementation comprises three embodiments which are describedhereafter, and said embodiments can be combined with one another forsaid preferred implementation of the device according to the invention.

According to a first preferred embodiment according to the invention,within the scope of said implementation, the device is such that thecomplementary reaction section is connected on the one hand to saiddistillation zone, at the height of a draw-off level, by a feed means,and connected on the other hand to said distillation zone in proximityto, generally substantially below, said level by a means intended forcirculation of the effluent of said complementary reaction section. Insaid first embodiment, the draw-off level is preferably situated betweenthe feed point of the distillation zone and the top of said distillationzone.

According to a second preferred embodiment according to the invention,within the scope of said implementation, the device is such that thecomplementary reaction section is connected on the one hand to saiddistillation-reaction zone, at the height of a draw-off level, by a feedmeans, and connected on the other hand to said distillation-reactionzone in proximity to, generally substantially below, said level by ameans intended for circulation of the effluent of said complementaryreaction section. In said second embodiment, the draw-off level ispreferably situated between the bottom of the distillation-reaction zoneand the bottom of the catalyst bed situated in the lowest part of saiddistillation-reaction zone.

According to a third preferred embodiment according to the invention,within the scope of said implementation, the device is such that thecomplementary reaction section is situated at least partly on the meansintended for circulation (as reflux) of the effluent from thedistillation-reaction zone to the distillation zone, i.e. the meansintended for circulation of the effluent from the distillation-reactionzone to, the distillation zone, one end of which is generally situatedat the bottom of the distillation-reaction zone, is connected to thecomplementary reaction section, and the means intended for circulationof the effluent from the complementary reaction section to thedistillation zone provides at least partly the reflux of thedistillation zone. There is possibly another means for circulating theeffluent directly between the bottom of the distillation-reaction zoneand the top of the distillation zone, providing at least another part ofthe reflux.

Generally, within the scope of said preferred implementation, whateverthe embodiment of the device according to the invention, the means forfeeding the complementary reaction section preferably comprises at leastone temperature control means allowing to adjust the temperature of theeffluent flowing into the complementary reaction section, usually inorder to obtain the maximum conversion in said complementary reactionsection.

Similarly, within the scope of said preferred implementation, whateverthe embodiment of the device according to the invention, the meansintended for circulation of the effluent of the complementary reactionsection preferably comprises at least one temperature control meansallowing to adjust the temperature of the effluent flowing out of thecomplementary reaction section, usually in order to recover the thermallevel of the zone into which the major part of said effluent flowsthereafter.

Besides, within the scope of said preferred implementation, whatever theembodiment of the device according to the invention, the complementaryreaction section comprises at least one additional reactant deliverymeans, usually in order to adjust the conversion in said complementaryreaction section.

The invention also relates to a process for preparing at least onetertiary alkyl ether by reaction of at least one aliphatic monoalcoholwith at least one olefin, comprising:

a) contacting in a reaction section at least one olefin and at least onealiphatic alcohol, generally in excess,

b) introducing, through at least one circulation means, the major partof the effluent of said reaction section into a distillation-reactionsection comprising a distillation zone and at least onedistillation-reaction zone, comprising at least one catalyst bed, saiddistillation-reaction zone being distinct from the distillation zone andproviding at least partly, preferably for the most part, most preferablypractically entirely, the reflux of the distillation zone,

c) drawing off, at the bottom of the distillation zone, tertiary alkylether and at the top of the distillation-reaction zone, an effluentcomprising unreacted hydrocarbons and residual aliphatic alcohol.

The process according to the invention is preferably such that the majorpart of an effluent drawn off at the top of the distillation zone is fedinto the distillation-reaction zone and the major part of an effluentthat is recovered at the bottom of the distillation-reaction zone is fedto the top of the distillation zone.

The process according to the invention is also preferably such that thereflux of the distillation-reaction zone is provided by drawing off, atthe top of the distillation-reaction zone, at least one effluent themajor part of which is condensed, part of said condensed effluent beingre-introduced at the top of the distillation-reaction zone and the otherpart of said condensed effluent forming mainly the part drawn off asdescribed at c) above.

A preferred implementation of the process according to the invention ispreferably such that said process is characterized in that saiddistillation-reaction section comprises at least one complementaryreaction section, comprising at least one catalyst bed.

Such a preferred implementation comprises three embodiments which aredescribed hereafter, and said embodiments can be combined with eachother in order to achieve said preferred implementation of the processaccording to the invention.

According to a first preferred embodiment according to the invention,within the scope of said preferred implementation, the process is suchthat the complementary reaction section is supplied with at least part,preferably with the most part of an effluent drawn off at the height ofa draw-off level of the distillation zone, the effluent of saidcomplementary reaction section being fed for the most part into saiddistillation zone in proximity to, generally substantially below saidlevel. In said first embodiment, the effluent is preferably drawn offbetween the feed point of the distillation zone and the top of saiddistillation zone.

According to a second preferred embodiment according to the invention,within the scope of said preferred implementation, the process is suchthat the complementary reaction section is fed with at least part,preferably with the most part of an effluent drawn off at the height ofa draw-off level of the distillation-reaction zone, the effluent of saidcomplementary reaction section being fed back into saiddistillation-reaction zone in proximity to, generally substantiallybelow said level. In said second embodiment, the effluent is preferablydrawn off between the bottom of the distillation-reaction zone and thebottom of the catalyst bed situated in the lowest part of saiddistillation-reaction zone.

According to a third preferred embodiment according to the invention,within the scope of said preferred implementation, the process is suchthat at least part, preferably the most part of the effluent circulatingfrom the distillation-reaction zone to the distillation zone in order toprovide at least partly the reflux of the distillation zone, passesthrough the complementary reaction section, i.e. at least part,preferably the most part of said effluent is fed into the complementaryreaction section, then the major part of the effluent of saidcomplementary reaction section is fed into the distillation zone.Another part of the effluent possibly circulates directly from thedistillation-reaction zone to the distillation zone.

Generally, within the scope of said preferred implementation, whateverthe embodiment of the device according to the invention, the temperatureof the effluent flowing into the complementary reaction section ispreferably controlled, which allows to adjust the temperature of theeffluent flowing from the complementary reaction section, usually inorder to obtain the maximum conversion in said complementary reactionsection.

Similarly, within the scope of said preferred implementation, whateverthe embodiment of the device according to the invention, the temperatureof the effluent flowing from the complementary reaction section andflowing into the distillation zone is preferably controlled, whichallows to adjust the temperature of the effluent flowing into thecomplementary reaction section, usually in order to recover the thermallevel of the zone into which the most part of said effluent flowsthereafter.

Besides, within the scope of said preferred implementation, whatever theembodiment of the device according to the invention, the complementaryreaction section is preferably supplied with additional aliphaticalcohol, which usually allows to optimize the ether(s) yield in saidcomplementary reaction section.

Finally, in the etherification process, the presence of thecomplementary reaction section, when there is one, advantageously allowsto control at best two significant parameters which are the temperatureand the addition of alcohol to said section, which favours optimizationof the ether(s) conversion or yield.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment according to U.S. Pat. No. 5,364,975.

FIG. 2 illustrates an embodiment according to U.S. Pat. No. 4,503,265.

FIG. 3 illustrates an embodiment according to Hydrocarbon Processing,March 1995, page 114.

FIGS. 4-9 illustrate embodiments in accordance with the invention.

The device according to the invention as shown in FIGS. 4, 5, 6, 7, 8and 9 comprises a fractionating column F1. The effluent at the bottom ofdistillation-reaction column F2 is used as the reflux of fractionatingcolumn F1, in which case (1), in the presence of finishing reactor R2,said reactor is fed by drawing off liquid from a draw-off level ofdistillation-reaction column F2, the effluent of said reactor R2 beingthen re-injected slightly below said level in column F2 (see FIG. 8); or(2) the effluent at the bottom of distillation-reaction column F2 isused as feed of finishing reactor R2 whose product is thereafter used asreflux of fractionating column F1 (see FIG. 5-7); or (3) said reactor isfed by drawing off liquid from distillation column F1, the effluent ofsaid reactor R2 being then re-injected slightly below said level intocolumn F1 (see FIG. 4).

The process according to the invention is more particularly a processfor preparing tertiary alkyl ether from isobutene and ethanol or otheraliphatic monoalcohols such as methanol, propanol or isopropanol, andfor preparing other tertiary alkyl ethers from C4, C5, C6 and C7iso-olefins and C1 to C4 aliphatic monoalcohols, in particular methanol,propanol and isopropanol. It thus relates more particularly to thepreparation of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether(ETBE) and tert-amyl methy ether (TAME). In all cases, the alcohol(s) is(are) generally in excess in relation to the olefin(s).

The feed used to prepare the tertiary alkyl ether according to theprocess of the invention generally contains at least one iso-olefincapable of reacting with the aliphatic monoalcohol in the etherificationreaction giving rise to tertiary alkyl ether.

Most often, but this is not essential, the feeds processed are cutsresulting from catalytic cracking and steam cracking afterfractionation. According to fractionation, these feeds can contain smallproportions of hydrocarbons whose number of carbon atoms per molecule isless or greater than that of the iso-olefins to be processed.

After reaction in reaction section R1, conducted under conventionalconditions (generally under the following operating conditions : in theliquid or mixed phase, at a pressure of 0.2to 3 MPa, preferably 0.5to 2MPa, at a temperature between 30 and 150° C., preferably between 40 and100° C.), the effluent flowing from reactor R1 generally containstertiary alkyl ether, the non-reactive hydrocarbons contained in thefeed, the non-converted hydrocarbons and the excess aliphaticmonoalcohol. This effluent is fed into the fractionating section whereit is distilled, generally under an absolute pressure of 0.3to 1 MPa andat a bottom temperature of 80 to 160° C.

The top effluent flowing from fractionating section F1 through line 1contains the non-reactive hydrocarbons of the feed, the non-convertedhydrocarbons and the excess aliphatic monoalcohol. This effluent isthereafter injected into distillation-reaction column F2. The bottomproduct of this distillation-reaction column is used as reflux offractionating column F1 and comprises non-reactive hydrocarbons,hydrocarbons that have not been converted in the first reaction sectionR1, the excess aliphatic monoalcohol and ether. The bottom effluentleaving distillation-reaction section F2 through line 7 contains thenon-reactive hydrocarbons of the feed, the non-converted hydrocarbons,aliphatic monoalcohol and ether.

The effluent drawn off at the bottom through line 9 comprises a tertiaryalkyl ether containing the lowest possible amount of monoalcohol(s).

A mixture of reactive and non-reactive hydrocarbons and of aliphaticmonoalcohol(s) carried over by azeotropy is recovered through line 2 atthe top of fractionating column F2.

The acid catalyst used in reactor R1, in the distillation-reaction zoneand in reactor R2 is generally selected from all the catalysts known tothe man skilled in the art to carry out the reaction considered. It canbe selected for example from sulfuric acid, hydrofluoric acid, aluminumchloride, boron fluoride, carbonaceous, sulfonated matter, such assulfonated carbon, sulfonated phenol-formaldehyde resins, sulfonatedcoumarone-indene polymers, or preferably sulfonatedstyrene-divinylbenzene copolymer resins. A zeolitic catalyst can also beused. It is also possible to use other compounds, notably mineralcompounds, comprising sulfonic groups (for example sulfonatedpolysiloxanes).

FIG. 4 diagrammatically shows an embodiment according to the inventionof the process for preparing tertiary alkyl ether wherein the effluentof etherification reaction section R1 is fed into a fractionating columnF1 where it is distilled, so as to produce at the bottom, through line9, a tertiary alkyl ether containing the lowest possible amount ofmonoalcohol(s) and, at the top, through line 1, a mixture ofhydrocarbons and of aliphatic monoalcohol carried along by azeotropy.This top product is fed into a distillation-reaction column F2. Thebottom product of this distillation-reaction column F2 flowing outthrough line 7 is then sent through pump P2 and line 8 as reflux offractionating column F1. In the embodiment of the process comprising afinishing reactor fed by drawing off liquid (line 16) from distillationcolumn F1, the effluent of this reactor is re-injected substantiallyslightly below the draw-off level in column F1 (line 19). The feed ofthis finishing reactor R2 is mixed with a complementary amount of atleast one aliphatic monoalcohol introduced through line 15. Thetemperature of this mixture is then adjusted in heat exchanger E2. Theproduct leaving this exchanger is thereafter fed into reactor R2 throughline 17. The effluent of this reactor is fed into heat exchanger E3through line 18 in order to adjust its temperature. The effluent of thisexchanger E3 is injected into the distillation-reaction column throughline 19. A liquid product is generally drawn off above the feed point ofthe distillation zone.

FIG. 5 diagrammatically shows another embodiment according to theinvention of the process for preparing tertiary alkyl ether comprising afinishing reactor on the reflux device of fractionating column F1 andwherein the temperature of the feed of finishing reactor R2 is adjustedby means of heat exchanger E2 and the temperature of the effluent ofthis reactor R2 is also adjusted by means of heat exchanger E2 and thetemperature of the effluent of this reactor R2 is also adjusted by meansof heat exchanger E3. The product leaving pump P2 is thus fed into heatexchanger E2 through line 8 and the effluent leaving this heat exchangerE2 through line 11 is used as feed of finishing reactor R2, and theeffluent leaving reactor R2 through line 13 is then fed into heatexchanger E3 through line 14 is then used as reflux of fractionatingcolumn F1. The other elements schematized in this figure are identicalto those described in connection with the previous figures.

FIG. 6 is diagrammatically shows another embodiment according to theinvention of the process wherein a complementary amount of at least onealiphatic monoalcohol, which can be the same alcohol as the alcohol usedin the main etherification reactor or a different monoalcohol, forexample selected from methanol, ethanol, propanol and isopropanol, isinjected through line 15 upstream from finishing reactor R2. The productleaving pump P2 through line 8 is thus mixed with an amount of at leastone aliphatic monoalcohol from line 15, and the mixture is fed intofinishing reactor R2. The effluent flowing from reactor R2 through line13 is thereafter used as reflux of fractionating column F1. The otherelements schematized in this figure are identical to those described inconnection with the previous figures.

FIG. 7 diagrammatically shows another embodiment according to theinvention of the process which combines the embodiments of FIGS. 5 and 6insofar as a complementary amount of at least one aliphatic alcohol isadded upstream from finishing reactor R2 through line 15 and thetemperature of the feed and of the effluent of this reactor is adjusted.According to this embodiment, the product leaving pump P2 through line 8is thus mixed with an amount of at least one aliphatic monoalcoholflowing in through line 15. This mixture is fed, through line 20, intoheat exchanger E2. The effluent flowing from this heat exchanger E2through line 11 is used as feed of finishing reactor R2. The effluentflowing out of reactor R2 through line 13 is then sent into heatexchanger E3. The effluent leaving this exchanger E3 through line 14 isthen used as reflux of fractionating column F1. The other elementsschematized in this figure are identical to those described inconnection with the previous figures.

FIG. 8 shows another embodiment of the process comprising a finishingreactor fed by drawing off liquid from distillation-reaction column F2,the effluent of this reactor being re-injected substantially slightlybelow the draw-off level of column F2. The feed of this finishingreactor R2 (line 16) is mixed with a complementary amount of at leastone aliphatic monoalcohol introduced through line 15. The temperature ofthis mixture is then adjusted in heat exchanger E2. The product leavingthis exchanger is thereafter fed into reactor R2 through line 17. Theeffluent of this reactor is fed into heat exchanger E3 through line 18in order to adjust the temperature thereof. The effluent of exchanger E3is injected into the distillation-reaction column through line 19.

FIG. 9 diagrammatically shows an embodiment according to the inventionof the process for preparing tertiary alkyl ether wherein the effluentof the etherification reaction section R1 is fed into a fractionatingcolumn F1 where it is distilled in order to produce, at the bottom,through line 9, a tertiary alkyl ether containing the lowest possibleamount of monoalcohol(s) and, at the top, through line 1, a mixture ofhydrocarbons and of aliphatic monoalcohol carried along by azeotropy.This top product is fed into a distillation-reaction column F2. Thebottom product of this distillation-reaction column F2 flowing outthrough line 7 is then sent back through pump P2 and line 8 as reflux offractionating column F1.

The following examples illustrate the invention. Examples 1, 2 and 3 arecomparative examples.

EXAMPLES 1 to 8

Ethyl tert-butyl ether (ETBE) was prepared on the one hand according tothe conventional pattern (patterns 1, 2 and 3 in connection with FIGS.1, 2 and 3 respectively) and on the other hand according to theembodiments described in the invention (patterns 4, 5, 6, 7, 8 and 9 inconnection with FIGS. 4, 5, 6, 7, 8 and 9 respectively).

The following table shows the composition of the feed and the isobuteneconversion in the main reaction section R1 and the operating conditionsimplemented for each pattern, in the finishing reactor when there isone, as well as the global conversion, the amount of ethanol obtained atthe bottom of fractionating column F1 and at the top ofdistillation-reaction column F2.

Operating conditions in the finishing reactor:

Catalyst: acid resin (sulfonated styrene-divinylbenzene copolymer)Reactor type: fixed bed Phase: liquid Pressure: 7.8.10⁵ Pa.Characteristics FIG. 1 FIG. 2 FIG. 3 FIG. 4 FIG. 5 FIG. 6 FIG. 7 FIG. 8FIG. 9 Isobutene content in the 22.9 22.9 22.9 22.9 22.9 22.9 22.9 22.922.9 feed entering reaction section R1 (% weight) Isobutene conversionin 92.3 92.3 92.3 92.3 92.3 92.3 92.3 92.3 92.3 reaction section R1 (%)Number of reaction 0 0 9 5 5 5 5 5 5 zones in distillation- reactioncolumn F2 Ethanol content in the 1.5 1.4 0.47 1.0 0.6 0.9 1.0 1.0 0.47raffinate of distillation- reaction column F2 (% by weight Ethanolcontent at the 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 3.5 bottom of thefractionating column (F1 or F2 FIG. 3) (% by weight) Ethanol/isobutenemolar 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 1.05 ratio at the inlet ofreaction section R1 Ethanol/isobutene molar 1.86 1.86 1.86 1.86 1.861.86 1.86 1.86 1.86 ratio at the inlet of fractionating column F1Ethanol/isobutene molar 0.8 2.0 ** 3.7 1.4 4.0 3.6 3.6 ** ratio at theinlet of finishing reactor R2 Ethanol/isobutene molar 1.05 1.05 1.051.08 1.05 1.08 1.08 1.08 1.05 ratio for the whole reaction unitTemperature at the inlet 57.2 68.6 ** 55 55 72.5 54.5 54.5 ** offinishing reacotr R2 (° C.) Temperature at the 59.5 69.8 ** 56.3 56.673.5 56.3 56.2 ** outlet of finishing reactor R2 (° C.) Global isobutene94.50 94.60 97.40 97.85 97.50 97.70 97.90 97.91 97.41 conversion (%)**Absence of finishing reactor

Implementation of the process surprisingly allows to increase accordingto the invention the global isobutene conversion with a catalytic columncontaining less reaction zones in the embodiments of examples 4 to 9(5reaction zones instead of 9 reaction zones). Knowing that a reactionzone in a column occupies about 3 meters in height, the height of thedistillation-reaction column can be decreased by about 12 meters, whichrepresents a sizeable investment decrease.

Furthermore, the additional reaction section, when there is one, isoutside the distillation column and can consequently be operated underoptimum and independent temperature and pressure conditions, thusallowing to maximize isobutene conversion. This can be seen for exampleby comparing the results obtained in FIG. 7 and those obtained in FIG.6.

What is claimed is:
 1. A process for preparing at least one tertiaryalkyl ether by reaction of at least one aliphatic monoalcohol and of atleast one olefin, comprising: a) contacting in a reaction section atleast one olefin and at least one aliphatic alcohol, b) introducing,through at least one circulating means, the major part of the effluentof said reaction section into a distillation-reaction section comprisinga distillation column and at least one distillation-reaction column,comprising at least one catalyst bed, said distillation-reaction columnbeing distinct from the distillation column and providing at leastpartly the reflux of the distillation column, c) drawing off, at thebottom of the distillation column, tertiary alkyl ether and at the topof the distillation-reaction column, an effluent comprising unreactedhydrocarbons and aliphatic alcohol.
 2. A process as claimed in claim 1,wherein the distillation-reaction column provides substantially thereflux of the distillation column.
 3. A process as claimed in claim 1,wherein said distillation-reaction section comprises at least onecomplementary reaction section, comprising at least one catalyst bed. 4.A process as claimed in claim 3, wherein said at least one complementaryreaction section is supplied with the most part of an effluent drawn offat the height of a draw-off level of the distillation column, theeffluent of said complementary reaction section being sent back to saiddistillation column in proximity to said level.
 5. A process as claimedin claim 4, wherein drawing off is performed between the feed point ofthe distillation column and the top of said column.
 6. A process asclaimed in claim 3, wherein the complementary reaction section issupplied with the most part of an effluent drawn off at the height of adraw-off level of the distillation-reaction column, the effluent of saidcomplementary reaction section being sent back to saiddistillation-reaction column in proximity to said level.
 7. A process asclaimed in claim 6, wherein drawing off is performed between the bottomof the distillation-reaction column and the bottom of the catalyst bedsituated in the lowest part of said distillation-reaction column.
 8. Aprocess as claimed in claim 3, wherein the major part of the effluentcirculating from the distillation-reaction column to the distillationcolumn to provide the reflux in the distillation column passes throughthe complementary reaction section, then the effluent of saidcomplementary reaction section is fed into the distillation column.
 9. Aprocess as claimed in claim 8, wherein the temperature of the effluententering the complementary reaction zone is controlled.
 10. A process asclaimed in claim 3, wherein the complementary reaction section issupplied with additional aliphatic alcohol.
 11. A process for preparingat least one tertiary alkyl ether by reaction of at least one aliphaticmonoalcohol and of at least one olefin, comprising: a) contacting in areaction section at least one olefin and at least one aliphatic alcohol,b) introducing, through at least one circulating means, the major partof the effluent of said reaction section into a distillation-reactionsection comprising a distillation column and at least onedistillation-reaction column, said at least one distillation-reactioncolumn comprising at least one catalyst bed, said distillation-reactioncolumn being distinct from the distillation column and providing atleast partly the reflux of the distillation column, c) drawing off, atthe bottom of the distillation column, tertiary alkyl ether and at thetop of the distillation-reaction column, an effluent comprisingunreacted hydrocarbons and aliphatic alcohol, wherein saiddistillation-reaction section comprises at least one complementaryreaction section, comprising at least one catalyst bed, and wherein thecomplementary reaction section is supplied with the most part of aneffluent drawn off at the height of a draw-off level of the distillationcolumn, the effluent of said complementary reaction section being sentback to said distillation column in proximity to said level, and whereindrawing off is performed between the feed point of the distillationcolumn and the top of said column.
 12. A process for preparing at leastone tertiary alkyl ether by reaction of at least one aliphaticmonoalcohol and of at least one olefin, comprising: a) contacting in areaction section at least one olefin and at least one aliphatic alcohol,b) introducing, through at least one circulating means, the major partof the effluent of said reaction section into a distillation-reactionsection comprising a distillation column and at least onedistillation-reaction column, said at least one distillation-reactioncolumn comprising at least one catalyst bed, said distillation-reactioncolumn being distinct from the distillation column and providing atleast partly the reflux of the distillation column, c) drawing off, atthe bottom of the distillation column, tertiary alkyl ether and at thetop of the distillation-reaction column, an effluent comprisingunreacted hydrocarbons and aliphatic alcohol, wherein saiddistillation-reaction section comprises at least one complementaryreaction section, comprising at least one catalyst bed, and wherein thecomplementary reaction section is supplied with the most part of aneffluent drawn off at the height of a draw-off level of thedistillation-reaction column, the effluent of said complementaryreaction section being sent back to said distillation-reaction column inproximity to said level, and wherein drawing off is performed betweenthe bottom of the distillation-reaction column and the bottom of thecatalyst bed situated in the lowest part of said distillation-reactioncolumn.
 13. A process as claimed in claim 1, wherein the major part ofan effluent drawn off at the top of the distillation column is fed intothe distillation-reaction column, and the major part of an effluentrecovered at the bottom of the distillation-reaction column is sent tothe top of the distillation column.
 14. A process as claimed in claim 1,further comprising providing reflux for the distillation-reaction columnby drawing off, at the top of the distillation-reaction column, at leastone effluent the major part of which is condensed, part of saidcondensed effluent being sent back to the top of thedistillation-reaction column as reflux for the distillation-reactioncolumn and the other part of said condensed effluent forming the drawnoff effluent described at c) in claim
 1. 15. A process as claimed inclaim 4, wherein the temperature of the effluent leaving thecomplementary reaction section and entering the distillation column iscontrolled.
 16. A process for reacting at least one aliphaticmonoalcohol and at least one olefin comprising: a) contacting in areaction section at least one olefin and at least one aliphaticmonoalcohol, b) introducing effluent from said reaction section into adistillation column of a distillation-reaction section, wherein saiddistillation-reaction section also has at least onedistillation-reaction column having at least one catalyst bed, and saidat least one distillation-reaction column is distinct from saiddistillation column, c) circulating effluent from said distillationcolumn to said at least one distillation-reaction column, d) passingfluid from the bottom of said at least one distillation-reaction columnto provide, at least partly, reflux to the top of said distillationcolumn, e) supplying at least one complementary reaction section havingat least one catalyst bed with fluid from either said distillationcolumn or from said at least one distillation-reaction column, anddelivering fluid from said at least one complementary reaction sectionto said distillation column or said at least one distillation-reactioncolumn, and f) drawing off product from the bottom of said distillationcolumn.